Problem: There are 84 trees on the banks of the river. The left bank has 39 of them. How many trees are on the right bank of the river?
Explanation: The number of trees on the right bank of the river is the difference between the number of trees on both banks of the river and the number of trees on the left bank. The difference is $84 - 39$ trees. $84 - 39 = 45$ trees.